The new generation of UHF Satellite Communications Standards (MIL-STD-188-181C, MIL-STD-188-182B, and MIL-STD-188-183B) require (up to) 54 different modems to simultaneously reside in the SATCOM terminal. These standards include the DAMA/IW (Demand-Assigned Multiple Access/Integrated Waveform) communications standards which are intended to increase existing satellite communications throughput by a factor of two. Designing the modems for the DAMA/IW standard requires new techniques to combine subfunctions of the modem to reduce software complexity and decrease development and integration time. Indeed, various satellite communication systems may have limited program space, CPU computation speed, and battery power.
In digital communications systems such as cellular and PCS (personal communications systems), computer communications systems, and SATCOM (satellite communications) systems, digital data is modulated by a modem onto a signal to be transmitted over a communications channel. Data is typically encoded before transmission to a receiver or to a storage device, to protect the data from errors which may result from a noisy communications channel or a defect in the storage medium. An encoder manipulates data symbols in accordance with an error correction code and produces error correction symbols or a structured redundancy output sequence. When the code word is later received or retrieved it is decoded to reproduce the data symbols, and errors in the data symbols are corrected, if possible, using the error correction symbols or the structured redundancy of code.
Continuous phase modulation (CPM) is being applied in communications due to its bandwidth efficiency and constant envelope characteristics. With CPM, the modulated signal phase transitions are smoothed. For example, with binary phase shift keying (BPSK) a logic one is transmitted as one phase of a modulated signal and a logic zero is transmitted as a 180-degree shifted phase with a sharp transition in phase. This sharp phase transition results in broadening of the transmitted spectrum. With CPM the phase of the transmitted signal makes smooth phase changes over the bits of the modulating digital signal. An example of CPM is minimum shift keying (MSK) modulation.
Multi-h continuous phase modulation (multi-h CPM) is itself a broad class of modulated waveforms. The class includes signals with constant amplitude but varying phase. Multi-h CPM differs from the single-h format by using a set of H modulation indices in a cyclic manner. This results in delayed merging of neighboring phase trellis paths and ultimately, in improved error performance. A detailed description of multi-h CPM waveforms, is included in the book “Digital Phase Modulation” by Anderson, Aulin, and Sundberg, Plenum Press, New York, 1986.
DAMA is a technique that increases the amount of users that a limited “pool” of satellite transponder space can support. The ability to share bandwidth is based on the theory that not all users will require simultaneous access to communication channels. DAMA systems quickly and transparently assign communication links or circuits based on requests issued from user terminals to a network control system. When the circuit is no longer in use, the channels are immediately returned to the central pool, for reuse by others. By using DAMA, many subscribers can be served using only a fraction of the satellite resources required by dedicated, point-to-point single-channel-per-carrier networks, thus reducing the costs of satellite networking.
Existing MIL-STD-188-183 and 183A terminals require acquisition and demodulation of various Phase Shift Keying (PSK) modulation types, such as Shaped Offset Quadrature Phase Shift Keying (SOQPSK), Differential Encoded Quadrature Phase Shift Keying (DEQPSK), and Binary Phase Shift Keying (BPSK) modulation types. New MIL-STD-188-181C (Integrated Waveform) requires acquisition and demodulation of Continuous Phase Modulation (CPM) types. The specified preamble phasing sequence for each of the modulation types is similar. The required Signal-to-Noise Ratio (SNR) requires advanced signal processing techniques to recover symbol frequency offset, phase offset, and timing.
Existing DAMA terminals and controllers acquire the modulation preamble by predefining the modem baud rate and correlating for the specific modem phasing pattern and start-of-message bit sequence. Baud is a measure of the bit rate, i.e. the number of distinct symbolic changes (signaling event) made to the transmission medium per second in a digitally modulated signal. As each symbol may stand for more than one bit of information, the amount of information sent per second is the product of the rate in baud and the number of bits of information represented by each symbol. The baud rate is equal to the symbol rate times the number of bits per symbol.
Existing DAMA terminals cannot auto baud-detect the modem baud rate (symbol rate). They have to be specifically set up for the expected modulation type and baud rate. Existing DAMA terminals acquire signal parameters in a low SNR conditions. Thus, there is a need for a signal processing approach that can acquire and demodulate both a CPM and PSK modulation type signal in a DAMA communication terminal with increased flexibility.